LTC3604 Linear Technology, LTC3604 Datasheet - Page 16

• Manufacturer Part Number: LTC3604
• Description: 2.5A 15V Monolithic Synchronous Step-Down Regulator
• Manufacturer: Linear Technology

LTC3604
APPLICATIONS INFORMATION
3. Other “hidden” losses such as transition loss, cop-
Thermal Considerations
The LTC3604 requires the exposed package backplane
metal (PGND pin on the QFN, SGND pin on the MSOP
package) to be well soldered to the PC board to provide
good thermal contact. This gives the QFN and MSOP
packages exceptional thermal properties, compared to
other packages of similar size, making it diffi cult in normal
operation to exceed the maximum junction temperature
of the part. In many applications, the LTC3604 does not
dissipate much heat due to its high effi ciency and low
thermal resistance package backplane. However, in applica-
tions in which the LTC3604 is running at a high ambient
temperature, high input voltage, high switching frequency,
and maximum output current, the heat dissipated may
exceed the maximum junction temperature of the part. If
the junction temperature reaches approximately 150°C,
16
Each time a power MOSFET gate is switched from low
to high to low again, a packet of charge dQ moves
from V
out of INTV
control bias current. In continuous mode, I
= f(Q
of the internal top and bottom power MOSFETs and f
is the switching frequency. For estimation purposes,
(Q
To calculate the total power loss from the LDO load,
simply add the gate charge current and quiescent cur-
rent and multiply by V
per trace resistances, and internal load currents can
account for additional effi ciency degradations in the
overall power system. Transition loss arises from the
brief amount of time the top power MOSFET spends in
the saturated region during switch node transitions. The
LTC3604 internal power devices switch quickly enough
that these losses are not signifi cant compared to other
sources.
Other losses, including diode conduction losses during
dead time and inductor core losses, generally account
for less than 2% total additional loss.
T
P
+ Q
LDO
T
+ Q
IN
B
= (I
) on the LTC3604 is approximately 1nC.
to ground. The resulting dQ/dt is a current
B
CC
GATECHG
), where Q
that is typically much larger than the DC
+ I
IN
T
Q
:
) • V
and Q
IN
B
are the gate charges
GATECHG
both power switches will be turned off until temperature
decreases approximately 10°C.
Thermal analysis should always be performed by the user
to ensure the LTC3604 does not exceed the maximum
junction temperature.
The temperature rise is given by:
where P
is the thermal resistance from the junction of the die to
the ambient temperature.
Consider the example in which an LTC3604EUD is operat-
ing with I
and an ambient temperature of 25°C. From the Typical
Performance Characteristics section the R
switch is found to be nominally 130mΩ while that of the
bottom switch is nominally 100mΩ yielding an equivalent
power MOSFET resistance R
R
From the previous section, I
1MHz, and the spec table lists the maximum I
Therefore, the total power dissipation due to resistive
losses and LDO losses is:
The QFN 3mm × 3mm package junction-to-ambient thermal
resistance, θ
temperature of the regulator operating in a 25°C ambient
temperature is approximately:
Remembering that the above junction temperature is
obtained from an R
junction temperature based on a higher R
increases with temperature. Redoing the calculation assum-
ing that R
temperature of 66°C. If the application calls for a signifi -
cantly higher ambient temperature and/or higher switching
frequency, care should be taken to reduce the temperature
rise of the part by using a heat sink or air fl ow.
DS(ON)
T
P
P
T
RISE
J
D
D
= 0.680W • 45°C/W + 25°C = 56°C
= I
= (2.5A)
TOP • 1.8/12 + R
D
= P
OUT
SW
OUT
is the power dissipated by the regulator and θ
D
2
JA
increased 15% at 56°C yields a new junction
θ
• R
= 2.5A, V
2
JA
, is around 45°C/W. Therefore, the junction
• (0.105Ω) + 12V • 2mA = 680mW
SW
DS(ON)
+ V
IN
IN
DS(ON)
at 25°C, we might recalculate the
= 12V, f = 1MHz, V
• (I
SW
GATECHG
GATECHG +
BOT • 10.2/12 = 105mΩ.
of:
www.DataSheet4U.com
is ~1mA when f =
I
Q
DS(ON)
)
DS(ON)
Q
OUT
to be 1mA.
of the top
since it
= 1.8V,
3604f
JA